(1) Field of the Invention
The present invention relates to glass substrates for use in liquid crystal display devices using thin film transistors (TFT), solar cells electroluminescence display devices and/or other devices having a thin film formed on the substrate, and in particular, to such a substrate having an improved chemical resistance.
(2) Description of the Prior Art
As a glass for use in solar cells which is chemically, electrically, and physically compatible with silicon deposited on the substrate by the vapour deposition and which, in particular, has a linear thermal expansion coefficient closely matching that of silicon over entire temperature range from the vapour deposition temperature down to room temperature while, being chemically inert with respect to the materials used in the vapour deposition process, U.S. Pat. No 4,180,618 (Reference 1) by J. W. Alpha et.al. assigned to Corning Glass Works discloses a glass substrate which is an alkaline earth metal aluminosilicate glass consisting essentially, by weight, of about 55-75% SiO.sub.2, 5-25% Al.sub.2 O.sub.3, and at least one alkaline earth metal oxide selected from the group consisting of 9-15% CaO, 14-20% SrO, 18-26% BaO, and mixtures thereof in a total amount equivalent on a molar bases to 9-15% CaO.
U.S. Pat. No. 4,634,683 (Reference 2) and Ser. No. 4,634,684 (Reference 3), both being issued to W. H. Dumbaugh, Jr. and assigned to Corning Glass Works disclose glass substrates for use in liquid crystal display devices using TFT, which are free from alkali metal ions and are transparent, flat, smooth, inert, compatible with silicon, in terms of thermal expansion, and capable of withstanding processing temperature of at least 850.degree. C.
The substrate glass disclosed in Reference 2 consists essentially of extremely fine-grained crystals homogeneously dispersed in a glass matrix, the base glass therefor being essentially free from alkali metal oxide and consisting essentially, in mole percent, of 2-6% BaO, and/or SrO, 18-26% Al.sub.2 O.sub.3, and 68-80% Si0.sub.2.
The substrate of the Reference 3 is prepared from a strontium aluminosilicate glass consisting essentially, in mole percent, of about 9-12% SrO, 9-12% Al.sub.2 O.sub.3, and 77-82% SiO.sub.2.
Generally speaking, a thin film such as silicon, after deposited on a substrate, is formed into a desired pattern by means of the photo etching to produce an operable device. In the photo etching process, various acids such as sulfuric acid, hydrofluoric acid and/or others and alkali solutions are used. Therefore, the substrate is required to have a chemical resistance to those acids and alkali solutions.
In References 1-3, the substrates disclosed therein are chemically inert to materials used in the vapour deposition process, but References 1-3 are silent as to whether or not those substrates are inert to materials used in the photo etching process.
The substrates disclosed in References 1 and 2 and the almost all substrates disclosed in Reference 3 contain a comparatively large amount of Al.sub.2 O.sub.3, such as more than 15% by weight. Al.sub.2 O.sub.3 tends to react with ammonium ions and hydrofluoric ions of buffered hydrofluoric acid (which is hydrofluoric acid to which ammonium fluoride is added as a buffer) used in the photo etching process, and the surface of the substrate is clouded in white. It is impossible to remove the white cloud by cleaning with water or other solutions. Although the white cloud can be rubbed off by, for example, sponge, the surface of substrate is injured.
Some of substrates disclosed in References 1 and 3 includes MgO which also reacts with ammonium ions and hydrofluoric ions to cloud the substrate surface white.
Further, the aluminosilicate glass as disclosed in References 1-3 has a considerably high melting temperature and a high liquidus temperature. Therefore, it is quite difficult to form an article of a desired shape as well as to produce a homogeneous glass without bubbles, inclusions, stones, striae, and others. In order to resolve the difficulty, it may be considered to use PbO as a flux in the glass. But, PbO evaporates from the glass melt and pollutes the circumference.
Another known glass for such substrates is SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --BaO glass such as 7059 produced and sold by Corning Glass Works. Although the glass includes a reduced Al.sub.2 O.sub.3 and no MgO, an amount of SiO.sub.2 is small so that the glass is not so sufficient in acid resistance. Therefore, the glass reacts with acids, such as sulfuric acid and others used in the photo etching process and the substrate surface is also clouded white. The glass further does not have a sufficient high strain point and is not good in the heat resistance.